DNA Is The Stuff Of Life Phil McClean Septemeber 2005 The research of Gregor Mendel dramatically changed our perception of heredity. His conclusion that a trait was controlled by a particulate factor suggested that some physical entity existed that controlled heredity. Mendel’s 1st Law, the law of segregation, suggested the factor was somehow reduced when it was passed onto what we know now is the gamete. We also know that this reduction event occurs during meiosis. Mendel’s 2nd Law, the law of independent assortment, implied that each trait was controlled by a unique factor. As significant as the discoveries of Mendel were, they did not consider the actual physical entity that controls heredity. A separate set of conclusions, many based on simple empirical scientific observation, lead to the eventual determination that these factors reside on chromosomes, and that DNA was the heredity material. Genes Reside on Chromosomes From 1879-1892, Flemming, Strasburger, Waldeyer, van Beneden, and Weismann made significant contributions to our concepts of chromosomes. Flemming (1882) observed structures in the nucleus of salamanders that bound dye, and these structures had a string like appearance. He termed the structures chromatin (or colored substance). He also developed the concept of cell division that he later termed mitosis. The universality of this discovery is attributed to Strasburger who discovered the same process in plants. Waldeyer, in 1888, called the structures that divided during mitosis chromosomes (or colored bodies). Weismann made the very critical observation that sperm and egg cells contain exactly half the number of chromosomes. van Beneden further observed that when a sperm cell fertilizes an egg, the result is the diploid chromosome number found in cells that undergo mitosis. He also determined that the sperm and egg cell each contribute equal numbers of chromosomes to the zygote. This final result demonstrated a link between chromosome and inheritance because it was generally known that offspring receive traits from both of their parents. These concepts were echoed by Hugo deVries (1889) but from a different perspective. He had called the hereditary material pangenes. He felt the pangene was larger than a single chemical molecule, yet still invisible. All of the pangenes that controlled all of the organisms traits were found within the cell of a diploid organism. He further suggested that reproductive cells receive half of the pangenes and that when reproductive cells unite the diploid number of pangenes is restored. In retrospect, these conclusions accurately represented our current understanding of how genes are passed from generation to generation. But although completely accurate, deVries theory was somewhat forgotten when E.B. Wilson released his book The Cell in Development and Inheritance. Notice the relationship between the cell and inheritance in the title; the linkage we are so now familiar with was growing stronger. His book also discussed the relationship between chromosomes and heredity. Although the relationship was suggested, the linkage was most strongly demonstrated by Sutton and Boveri. Walter Sutton and Theodor Boveri worked independently. Boveri observed (as had van Beneden) that male and female gametes contribute an equal number of chromosomes to the zygote. In 1902, he made the link between chromosomes and the factors that were described by Mendel. This is most evident in a quote of his that states "… the characters dealt with in Mendelian experiments are truly connected to specific chromosomes." Sutton described chromosomes as unique individual units that occur in pairs that separate during meiosis. These concepts of Doveri and Sutton are known as the Chromosomal Theory of Inheritance. These studies set the stage for the intense study of chromosomes and their role in heredity. The final link between heredity and chromosomes required definitive proof that a genetic factor was indeed located on a chromosome. The work of Thomas Hunt Morgan, and later Calvin Bridges, provided such proof. Morgan, in 1910, discovered a mutant line of Drosophila with white rather than the wild type red eye. He suggested by genetic experiments that the gene for eye color was located on the X chromosome. This is one of the two sex chromosomes in the fly. Bridges followed this up by cytological experiments in 1914 that coupled the presence of the X chromosome with a specific eye color. This demonstrated conclusively that genes indeed reside on chromosomes. If chromosomes were the carriers of genetic information, then they must contain all of the factors that make an organism unique. Yet, the total number of chromosomes is vastly less than the number of traits. For chromosomes to actually contain the genetic material, they must be shown to contain many factors. This was conclusively shown by researchers in Morgan’s laboratory. By mating among Drosophila, it was shown that a chromosome contains multiple genes that could be organized into a linear linkage group. The discovery that the number of linkage groups that could be mapped in a species was equal to the number of chromosomes solidified the importance of chromosomes in heredity. It was clear that genes resided on chromosomes, and chromosomes were the functional unit of heredity. Alfred Sturtevant (1925) demonstrated that the actual physical order of the genes was also essential. He discovered the concept of position effect by showing that altering the physical environment of a gene on a chromosome altered its function. This implied that the actual physical organization of the chromosome was essential for proper phenotypic expression. Therefore, it became clear that we could not simply look at a single gene to understand its expression, but rather we needed to consider the entire chromosomal region. In many regards, this pointed the way to genomics, a science where we considered the entire DNA complement. DNA is the genetic material DNA is the stuff of life. Embedded in DNA are the sequences that encode for all of the genes that determine what an organism will look like and how that organism will react to its environment. But the discovery that DNA was the master information molecule was spread over nearly 80 years. Early (1866) observations of Ernst Haeckel suggested that nucleus transmitted hereditary information to the next generation. Because of his professional stature in biology, this concept focused the work of others toward the nucleus. The next milestone in determining the nature of the hereditary information was performed by Friedrich Miescher. He studied pus cells that he collected from bandages he collected from surgeries. The majority of these cells were white blood cells. The cells are distinctive because they are primarily composed of nuclei. In 1871, he called this nuclear material nuclein. Miescher further characterized this material chemically (1874). First, he determined that nuclein was rich in phosphorus. Further analysis revealed that it contained acidic (DNA and RNA) and basic (histone proteins) portions. Even though a link was made between a chemical molecule and the hereditary material, further discoveries were necessary to make a definitive link between DNA and heredity. The experiments of Fred Griffith (1928) set the stage for the final proof that DNA was the genetic material. Working with lethal and non-lethal strains of the Streptococcus pneumoniae, he was able to show that a factor from a lethal strain could convert a non-lethal strain to a lethal strain. Because his factor was able to convert one phenotype to another (the true nature of a gene), Griffith called this factor the transforming principle. Avery, MacLeod, and McCarty (1944) studied this factor in depth. Their experiments definitively proved that the factor was DNA, and not protein or RNA, the other two constituents in the nucleus. In retrospect, the Avery, MacLeod and McCarty experiments were definitive. Others did not think so, and the search continued. The experiments by Hershey and Chase (1952) demonstrated to many scientists that DNA was the hereditary material. Using T2 bacteriophage, they showed that DNA and not protein entered the bacterial cell. Since new T2 phage particles were produced inside the cell, it was concluded that DNA was responsible for its development. Although quite conclusive at the time, scientists now realize that the fact that the DNA was actually contaminated with a bit of protein make these less than conclusive experiments. Although we focus so much of our attention on DNA, it should be noted that RNA can also be the genetic material. But this is only the case for RNA viruses. Heinz Fraenkel-Conrat (1957) and researchers working with him performed these experiments. Using reconstruction experiments, they were able to able to interconvert strains of tobacco mosaic virus by the addition of the RNA isolated from an alternate strain. Now that it was known that DNA was indeed the genetic material, it only remained to determine its structure. That work was completed by James Watson and Francis Crick (1953). They determined that DNA is double-stranded, the strands are oriented in an anti-parallel manner to each other, purines nucleotides are opposite pyrimidines nucleotides in the strand, guanine hydrogen bonds with cytosine, while adenine hydrogen bonds with thymine, and the structure is stabilized by hydrogen bonds and hydrophobic bonding between stacked bases. It should be noted that Watson and Crick did not perform any experiments. Their